The present invention relates to pollution control devices, and particularly to catalytic converters and diesel particulate filters or traps for an automotive exhaust system. The pollution control devices typically comprise a metal housing with a monolithic element securely mounted within the casing by a resilient and flexible mounting mat. The mounting mat is comprised of an intumescent sheet material having inserts formed of a non-intumescent ceramic fiber composite.
Pollution control devices are universally employed on motor vehicles to control atmospheric pollution. Two types of devices are currently in wide spread use--catalytic converters and diesel particulate filters or traps. Catalytic converters contain a catalyst, which is typically coated on a monolithic structure mounted in the converter. The monolithic structures are typically ceramic, although metal monoliths have been used. The catalyst oxidizes carbon monoxide and hydrocarbons, and reduces the oxides of nitrogen in automobile exhaust gases to control atmospheric pollution. Due to the relatively high temperatures encountered in these catalytic processes, ceramics have been the natural choice for catalyst supports. Particularly useful catalyst supports are provided by ceramic honeycomb structures as described, for example, in U.S. Pat. No. Re. 27,747.
More recently, catalytic converters utilizing metallic catalyst supports (metallic monoliths) have also been used for this purpose. (See for example, in U.K. Patent No. 1,452,982, U.S. Pat. No. 4,381,590 and SAE paper 850131).
The most common diesel particulate filters or traps are monolithic wall-flow filters. These monolithic wall-flow type diesel particulate filter elements are typically comprised of honeycombed, porous, crystalline ceramic (e.g., cordierite) material. Alternate cells of the honeycombed structure are typically plugged such that exhaust gas enters in one cell and is forced through the porous wall of one cell and exits the structure through another cell. The size of the diesel particulate filter element depends on the particular application needs. Useful diesel particulate filter elements are commercially available, for example, from Corning Inc. of Corning, New Your, and NGK Insulator Ltd. of Nagoya, Japan. Useful diesel particulate filter elements are discussed in "Cellular Ceramic Diesel Particulate Filter," Howitt et al., Paper No. 810114, SAE Technical Paper Series, 1981.
In the state of the art construction of these devices, each type of device has a metal housing which holds within it a monolithic structure or element that can be metal or ceramic, and is most commonly ceramic. The monolithic structure is mounted in the housing in a process referred to as canning. There is a gap or space between the monolith and the housing which varies because there is a range of size tolerances for both the monolith and the housing. The largest gap exists when the monolith is on the small end of the range and the housing is on the large end of the range. To avoid damage to the monolith and to hold it in place a mounting material, such as an intumescent mounting mat or an intumescent paste, is typically disposed around the monolith before canning. The mounting material fills the gap. After the wrapped monolith is inserted into the housing, the can is pressed closed and flanges along the lateral edges of the housing are welded. After installation on the vehicle, the pollution control device is heated by the hot exhaust gases which expand the intumescent materials generating additional holding pressure. The amount of pressure is determined by the mount density of the materials and the temperatures of use. If the mount density is too low, there will be insufficient pressure to hold the monolith in place. If the mount density is too high, excessive pressure can be exerted by the mounting material between the housing and the monolith causing deformation of the housing and/or damage to the monolith.
After the monolith has been secured in the housing, the intumescent mounting material serves to avoid or reduce damage from other conditions that can be problematic to the pollution control device. The device can be subjected to damaging vibrations both before and after installation in a vehicle. Additionally, the entire device is subjected to elevated temperatures, e.g., over 300.degree. C., for various periods of time.
A ceramic monolith has a coefficient of thermal expansion generally an order of magnitude less than the metal (usually stainless steel) housing in which it is contained, so that at elevated temperatures, the mounting materials must expand sufficiently to compensate for the differential expansion, but not so much as to create excessive pressure which can damage the housing or the monolith. The mounting material also prevents hot exhaust gases from passing between the monolith and the metal housing (thereby bypassing the catalyst).
Typically, the mounting materials include inorganic binders, inorganic fibers that may also serve as a binder, intumescent materials, and optionally, organic binder, fillers and other adjuvants. The materials are used as pastes, sheets, and mats. Ceramic mat materials, ceramic pastes, and intumescent sheet materials useful for mounting the monolith in the housing are described in, for example, U.S. Pat. Nos. 3,916,057 (Hatch et al.), 4,305,992 (Langer et al.), 4,385,135 (Langer et al.), 5,254,416 (Langer et al.), 5,242,871 (Hashimoto et al.), 3,001,571 (Hatch), 5,385,873 (MacNeil), 5,207,989 (MacNeil), and Great Britain Patent 1,522,646 (Wood).
U.S. Pat. No. 4,999,168 to TenEyck describes a crack resistant intumescent sheet having a preformed intumescent layer adhesively bonded to a reinforcing layer of a sheet material such as craft paper, plastic film, inorganic fabric.
U.S. Pat. No. 4,865,818 to Merry et al. describes a method of producing a catalytic converter by wrapping a thin sheet of mat material around the monolith at least twice in a layer wise fashion.
U.S. Pat. No. 4,929,429 to Merry describes a composite for catalytic converters having a ceramic fiber mat stitched-bonded to an intumescent mat material.
U.S. Pat. No. 4,048,363 to Langer et al. describes a composite having at least two layers of similar sheets of intumescent materials.
As the pollution control device is cycled between high and low temperatures, the size of the gap between the monolith (metal or ceramic) and the housing continuously changes, and the mounting mat is repeatedly compressed and uncompressed. In cases where the housing reaches very high temperatures, i.e., greater than about 700.degree. C., deformation of the housing can occur. In these cases, conventional intumescent mat mounting material may lack the high temperature resiliency to provide continued support for the monolith. A need thus exists for a mounting system which is sufficiently resilient and compressible to accommodate the changing gap between the monolith and the metal housing without causing deformation of the metal housing. Further, while the state of the art mounting materials have their own utilities and advantages, there remains an ongoing need to improve mounting materials for use in pollution control device. Additionally, it would be desirable to provide materials that function well over a broader temperature range.